Passive thermal stabilization of the optical path length in a tunable laser

ABSTRACT

The present invention provides an apparatus for passively stabilizing the optical pathlength in tunable lasers. Lasers stabilized using the passive stabilization apparatus exhibit reduced mode hop and increased wavelength stability during temperature variations of the laser or surrounding environment. The stabilization makes the laser suitable for a broad range of applications including: optical signal generators and optical multimeters.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority from Provisional Application Number:60/099,901, entitled “Modulation/Continuous Wave Constant Power ControlCircuit”; No. 60/100,055, entitled “Drive Train Passive ThermalCompensation”; No. 60/099,839, entitled “Phase Continuous Tuning in AnExtended Cavity Diode Laser Using Dispersion Compensation Together WithMechanical Grounding”; No. 60/099,865, entitled “Drive Train Flexure”;and No. 60/099,831, entitled “Passive Thermal Compensation of ExternalCavity Diode Laser”, all filed Sep. 11, 1998. Each of the above-citedapplications is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention generally relates to optical multimeters and moreparticularly to signal generating portions thereof.

2. Description of the Related Art

The telecommunications network serving the United States and the rest ofthe world is presently evolving from analog to digital transmission withever increasing bandwidth requirements. Fiber optic cable has proved tobe a valuable tool, replacing copper cable in nearly every applicationfrom large trunks to subscriber distribution plants. Fiber optic cableis capable of carrying much more information than copper with lowerattenuation.

The T-1 standards committee ANSI has provided a draft document, “ANSIT1.105-1988”, dated Mar. 10, 1988, which sets forth specifications forrate and format of signals which are to be used in optical interfaces.The provided specifications detail the Synchronous Optical Network(SONET) standard. SONET defines a hierarchy of multiplexing levels andstandard protocols which allow efficient use of the wide bandwidth offiber optic cable, while providing a means to merge lower level DSO andDS1 signals into a common medium. In essence, SONET established auniform standardization transmission and signaling scheme, whichprovided a synchronous transmission format that is compatible with allcurrent and anticipated signal hierarchies. Because of the nature offiber optics, expansion of bandwidth is easily accomplished.

Currently this expansion of bandwidth is being accomplished by what isknown as “wavelength division multiplexing” (WDM), in which separatesubscriber/data sessions may be handled concurrently on a single opticfiber by means of modulation of each of those subscriber datastreams ondifferent portions of the light spectrum. WDM is therefore the opticalequivalent of frequency division multiplexing (FDM). Currentimplementations of WDM involve as many as 128 semiconductor lasers eachlasing at a specific center frequency within the range of 1525-1575 nm.Each subscriber datastream is optically modulated onto the output beamof a corresponding semiconductor laser. The modulated information fromeach of the semiconductor lasers is combined onto a single optic fiberfor transmission. The data structure of a basic SONET signal at atypical data rate of 51.84 Mbps, a.k.a. an STS-1 signal, has 9 rows of90 columns of 8 bit bytes at 125 μs frame period. The first threecolumns of bytes in the SONET signal are termed the transport overhead(TOH) bytes that are used for various control purposes. The remaining 87columns of bytes constitute the STS-1 synchronous payload envelope(SPE). As this digital signal is passed across a SONET network, it willbe subject at various intervals to amplification by, for example, Erbiumdoped amplifiers and coherency correction by, for example, opticalcirculators with coupled Bragg filters. At each node in the network,e.g. central office or remote terminal, optical transceivers mounted onfiber line cards are provided. On the transmit side, a framer permitsSONET framing, pointer generation and scrambling for transmission ofdata from a bank of lasers and associated drivers, with each laserradiating at a different wavelength. On the receive side, the incomingsignals are detected by photodetectors separated into channels, framedand decoded.

As more and more optical signal equipment (transmitting, receiving,amplification, coherence and switching) is being designed and utilized,a need has arisen for optical multimeters, e.g. signal generators anddetectors, which can be used to test the various components of anoptical, e.g. SONET, network. What is needed is a tunable optical signalgenerator that does not require the complex control systems relied on byprior art devices. Those control systems utilize closed loop feedback ofwavelength or position to select the output wavelength of the opticalsignal generator. As a result they are expensive and exhibit a largeform factor.

SUMMARY OF THE INVENTION

The present invention provides an apparatus for passively stabilizingthe optical pathlength in tunable lasers. Lasers stabilized using thepassive stabilization apparatus exhibit reduced mode hop and increasedwavelength stability during temperature variations of the laser orsurrounding environment. The stabilization makes the laser suitable fora broad range of applications including optical signal generators andoptical multimeters.

In an embodiment of the invention, a tunable laser with a base, a gainmedium, a tunable feedback device and a first compensating element isdisclosed. The gain medium and the tunable feedback device are coupledto the base. The tunable feedback device provides feedback of a selectedwavelength to the gain medium. The first compensating element providescoupling to the base for at lease one of the gain medium and the tunablefeedback device such that thermal expansion of the compensating elementmaintains a substantially constant integer number of half-wavelengthswithin a resonant cavity defined by the gain medium and the tunablefeedback device during temperature variations in the tunable laser.

In another embodiment of the invention, a tunable laser with a base, again medium, a first and second feedback device, a pivot arm and a firstcompensating element is disclosed. The gain medium, and first and secondfeedback devices are coupled to the base. The first feedback deviceprovides feedback of a selected wavelength to the gain medium. The pivotarm has a proximal and distal end. At the proximal end the pivot armpivotally attaches to the base at a first pivot axis. The secondfeedback device couples to the distal end of said pivot arm to providefeedback of the selected wavelength to said first feedback device. Thesecond feedback device together with the first feedback device and thegain medium define a resonant cavity. The second feedback deviceresponds to the arcuate displacement of the pivot arm to vary theselected wavelength. The first compensating element couples the secondfeedback device to the distal end of said pivot arm such that thermalexpansion of the first compensating element maintains a substantiallyconstant integer number of half-wavelengths within the resonant cavity.

Other aspects and advantages of the invention will become apparent fromthe following detailed description, taken in conjunction with theaccompanying drawings, illustrating by way of example the principles ofthe invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be readily understood by the followingdetailed description in conjunction with the accompanying drawings,wherein like reference numerals designate like structural elements, andin which:

FIG. 1 shows an optical multimeter, according to the current invention,coupled to an optical network.

FIG. 2 is a hardware block diagram of an embodiment of the opticalmultimeter according to the current invention.

FIG. 3 is an isometric view of the signal generator portion of theoptical multimeter shown in FIG. 2 which incorporates a tunable laser.

FIG. 4 is a top plan view of the tunable laser shown in FIG. 3.

FIG. 5 is an exploded isometric view of a tunable laser shown in FIGS.2-4.

FIG. 6 is an assembled view of the tunable laser shown in FIG. 5.

FIG. 7 is an exploded isometric view of the drive portion of the tunablelaser shown in FIGS. 3-4.

FIG. 8 is an assembled view of the drive portion of the laser shown inFIG. 7.

FIG. 9 is an isometric view showing the laser and actuator portions ofthe tunable laser shown in FIGS. 3-4.

FIG. 10 is a hardware block diagram showing a manufacturing setupconfiguration for programming and calibrating the signal generatorportion of the optical multimeter.

FIGS. 11A-D are plan views of hardware associated with thermallystabilizing the optical pathlength of the laser cavity.

FIG. 12 is a top view of the resonant cavity portion of the tunablelaser shown in FIGS. 2-3 with compensating elements for thermallystabilizing the optical pathlength.

FIG. 13A is a top plan view of a prior art drive train for mechanicallyactivating the tuning element of a tunable laser.

FIGS. 13B-D are top plan views of alternate embodiments of hardware forthermally stabilizing the tuning element of a mechanically tuned laserin accordance with an embodiment of the current invention.

FIG. 14A is an isometric view of a mounting system for attaching bothintermediate and optical elements of a tunable laser to a base.

FIG. 14B is a cross-sectional side view of the mounting system shown inFIG. 14A.

FIG. 15 is a detailed circuit diagram of an embodiment of a modulationcircuit for driving the signal generator shown in FIG. 2.

FIG. 16 shows modulated waveforms generated by the signal generatorportion of the optical multimeter.

FIG. 17 shows a data lookup table utilized to configure the signalgenerator to output a beam at a selected wavelength.

FIG. 18 shows an embodiment of the processes associated with generatingthe lookup table.

FIG. 19 shows an embodiment of the processes associated with selectingan output wavelength for the signal generator.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides an optical multimeter for use incalibrating and testing the various components associated with anoptical network, e.g. SONET. The optical multimeter includes ananalog/digital signal generator for delivering an optical output beamwhich can be modulated over a wide range of frequencies, duty cycles andamplitudes with very precise definition of the rising and falling edgesof the waveform. Circuitry is also provided for modulation of an analogmodulation signal onto the optical output. The signal generator includesa tunable laser that is thermally stabilized as to optical path length,as well as tuning angle of the tuning element. This substantiallyreduces thermally induced mode hops as well as thermally inducedvariations in the output wavelength. The tunable laser exhibits a smallform factor due in part to a novel wavelength control process whichutilizes an open loop system to maintain precise output wavelengthcontrol without the requirement of either a wavelength or positionfeedback device. Additionally, the tunable laser incorporates aninexpensive modulator circuit which combines a low frequency closed looppower control with a separate digital modulator. A novel mountingmechanism is disclosed which simplifies device fabrication by allowingprecise positioning of optical and intermediate elements of the laser toa base.

FIG. 1 shows an optical multimeter 100 coupled via a network accessdevice 102 to the various components of an optical network 120. TheSynchronous Optical Network (SONET) standard defines a networkingapproach for high speed data communication at data rates from 51.8 Mbpsto 2.48 Gbps. With the implementation of SONET, communication carriersthroughout the world can interconnect their existing digital carrier andfiber optic systems.

A plurality of central offices/switching centers 104-106 are showncoupled to an optical network 120. Datastreams are multiplexed usingwavelength division multiplexing (WDM) in different portions of theoptical spectrum. The network itself typically includes: Erbium dopedline amplifiers 122-124 to maintain signal strength, circulators 126with Bragg filters to maintain the coherence of the signals, and opticalswitches to route the traffic between appropriate data terminals. At thecentral office, the line cards 108-112 handle the transmission andreception of datastreams. On the transmit side, each line card includessemiconductor lasers each tuned to a specific wavelength within therange of 1525-1575 nm. Optical modulators inject datastreams into theoutput beams of these lasers which are collimated in a single fiberoptic line for transmission across the network. On the receive side,each card includes photodetectors and demodulators to convert thereceived data into a format suitable for transmission across fibersubscriber lines 130 or copper subscriber lines 132 to data terminals114-116 or to a traditional analog telephone 118. All of thesecomponents need to be tested and calibrated across a range offrequencies and power levels with signals that may be analog or digital.The high precision optical multimeter of the current invention includesa high precision optical signal generator and optical detector whichallows these components to be tested on site or on the lab bench.

FIG. 2 shows an exterior isometric view of the optical multimeterhousing as well as a hardware block diagram of the components within theoptical multimeter. The optical multimeter 100 includes: display 200,user inputs 202, I/O interface 204, processor 206, memory 208,modulation circuit 222, overload sensor 242, temperature sensor 246,power-detector 270 and the signal generator 250. The signal generatorincludes: gain medium 224, tunable cavity 226, output 228, actuator 230and start condition detector 240. The memory 208 includes program code210 and lookup table 212.

The I/O interface couples the display 200 and user inputs 202 to thesystem bus 216. The memory 208 is coupled to the processor 206 and thesystem bus. The system bus also couples to the power detector 270,modulation circuit 222, start condition sensor 240, overload sensor 242,and temperature sensor 246. Within the signal generator the actuator 230drives a tuning element within the tunable cavity 226. The startcondition detector 240 couples either directly to the actuator or to thetunable element within the tunable cavity to detect a starting pointthereof.

On the transmit side, the signal generator 250 generates an output beam260. The output beam can be tuned to any of a number of centerwavelengths associated with, for example, each channel in the IEEE-ITUstandard. Selection of a center wavelength is accomplished by an openloop control system which utilizes the lookup table 212 to drive theactuator to the selected wavelength. Unlike prior art optical signalgenerators which require a continuous feedback across the tuning rangeof either position of the tuning element or wavelength of the output, nofeedback is required to select output wavelength. Instead, an open loopcontrol system is implemented, thus reducing the cost and form factor ofthe signal generator. To fabricate a precision tunable signal generatorwithout either a wavelength feedback apparatus or position sensor, theremust be a precise and repeatable correlation between the control signalssupplied by the processor to the actuator 230 and the output wavelength260. This in turn requires that the hardware be optically stable acrossa range of temperatures, where optical stability includes stability ofboth the optical pathlength as well as the tuning angle of the tuningelement within the tunable cavity. Each signal generator includesprocesses for utilizing a unique lookup table, the records of which aregenerated during the manufacture of the device to correlate drivesignals with output wavelength. This calibration involves ramping thetunable laser through a range of frequencies, and using a wavelengthsensor, recording the correlation between output wavelength and thedrive signals supplied to the actuator. This information is recorded ina wavelength_vs._drive signal lookup table 212 which is stored in memory208 during the assembly of the device. Responsive to a user request foran output signal centered within a specific channel, the processor 206,using this table, generates the required number of actuator signals totune the laser to the requested channel.

Environmental effects on laser output wavelength must be accounted for.Temperature is one of the primary environmental factors which affectoutput wavelength. The center wavelengths associated with neighboringchannels are narrowly separated, i.e. less than 1 nm apart. Thesewavelength variations could easily be produced by thermalexpansion/contraction of the tuning mechanism for the tunable cavity 226or by variations in the optical pathlength. Two techniques may be usedsingly or in combination to substantially reduce the effect oftemperature variations on the wavelength stability/accuracy of theoutput beam. One technique involves actively adding or removing energyfrom the cavity to maintain a constant thermal state, thus avoidingthermal contraction and expansion by stabilizing the temperature in thetunable cavity. The other involves fabricating the tunable laser in amanner which allows thermal expansion and contraction without, however,inducing temperature-related variation in the output wavelength from thetunable laser. Although either approach is suitable for use with thecurrent invention, the latter passive approach set forth in FIGS. 11-14,has the advantage of lower cost and form factor since no thermalgenerator, monitor, and control circuitry is required.

In operation, the optical multimeter may be used singly or incombination with other multimeters to test optical devices on the benchor across a network connection. One method for testing an optical deviceinvolves coupling the multimeter output beam 260 to a device under test(DUT) and monitoring the DUT output 262 at the power detector 270. For aDUT such as an Erbium doped optical amplifier, the output signal 260 canbe injected into the optical amplifier, and the resultant output 262signal from the amplifier may be coupled to the receive side of theoptical multimeter. On the receive side, an optical signal 262 receivedvia power detector 270 is digitally sampled and passed to the processorvia system bus 212. The processor executing program code 210 stored inmemory 208 analyzes the received signal according to parameters input bythe user on input 202. Additionally, the processor passes the signal viaI/O interface 204 for presentment to the user on display 200. Becausethe output signal 260 is precisely controlled, the processor 206 maycompare the received signal with the known parameters of the transmittedsignal in order to characterize various parameters of the DUT such as:power level, gain, rise and fall time, etc.

In an alternate embodiment of the invention both the signal generatorand the power detector as well as other modules would each implementplug-and-play technology with dedicated master processor resident in themultimeter mainframe.

FIG. 3 as an isometric view of a hardware embodiment of the signalgenerator 250 shown in FIG. 2. The base 300, fiber mount 302, fibercoupling 304, motor bracket 310, laser diode housing 330, diffractiongrating 340, grating mount 342, retroreflector 350, compensating element352, pivot bracket 354, actuator 370, drive train 376 and startcondition sensors 390-392 are shown. In an embodiment of the inventionthe signal generator incorporates a tunable laser in a Littman-Metcalfconfiguration. In this configuration, the laser diode within housing330, the diffraction grating 340 and the retroreflector 350 are laid outin a generally triangular arrangement. The laser housing 330 is affixedto base 300 at a grazing angle with respect to the diffraction grating340, such that a reflection from the diffraction grating passes to thefiber coupling 304 where it is coupled to a fiber optic (not shown). Thediffraction grating is coupled to the grating mount 342, which is inturn fastened to the base 300. The fiber coupling 304 is fastened to thefiber mount 302, which is in turn coupled to base 300. The laser beam isalso diffracted from the diffraction grating 340 striking theretroreflector 350. The return beam from the retroreflector strikes thediffraction grating and returns through an anti-reflective coating onthe front facet of the laser diode within housing 330 to select theoutput wavelength of the laser. The retroreflector 350 is coupled to acompensating element 352, which is in turn coupled to the pivot bracket354. The pivot bracket is coupled to the base 300 at a pivot point whichallows tuning of the laser by combined rotation and translation of theretroreflector with respect to the diffraction grating (See FIGS. 5-6).The pivot point may be selected to provide the requisite combination ofrotation and translation so as to maintain a constant integer number ofhalf-wavelengths in the cavity, thus reducing mode hopping. This pivotpoint may be selected in accordance with the teachings of U.S. Pat. No.5,319,668, issued on Jun. 7, 1994, entitled “Tuning system for ExternalCavity Diode Laser” and having in common with the current invention theAssignee New Focus Inc., of Santa Clara, Calif.

In an embodiment of the invention, a pivot bracket and attachedretroreflector is used to tune the laser. The motion of the pivotbracket is brought about by a linear translation of the drive train 376coupled to a pivot arm to which the pivot bracket is attached. Themotion of the pivot arm results from the actuator 370. The actuator 370is coupled to the motor bracket 310, which is in turn coupled via afastener placed within coupling 312 to the base 300. In this embodimentof the invention, the actuator is a rotary stepper motor. Otheractuators may be used with equal advantage, including, but not limitedto: linear stepper motors, piezo-electric stacks, bimetallic elements,AC/DC motors, etc. As will be obvious to those skilled in the art, theactuator 370 could be bolted directly to the base 300 without departingfrom the scope of the invention. The stepper motor operates undercontrol of the processor 206 (See FIG. 2). In an embodiment of theinvention, start condition sensors 390-392 are used to determine astarting position for the drive train by making a linear and arcuatereadout of the drive train. These sensors, in combination with thewavelength lookup table 212, allow the processor to control the actuatorso as to select output wavelengths for the tunable laser (See FIG. 9).

FIG. 4 shows a top plan view of the tunable laser embodiment shown inFIG. 3. The base 300 with attached laser diode housing 330, diffractiongrating 340, and fiber coupling 304 is shown. The actuator 370 iscoupled to the base 300 via motor bracket 310 and strap 440. Theindividual components of the drive train 376 are visible and include:drive shaft 400, hub and rim 402-404, rotary flex member 406,compensating element 410, translation unit 412, cylindrical nut 414,lead screw 418, and linear flex member 420.

The drive train 376 comprises rotary, linear, and arcuate portions.Generally the drive shaft converts the rotary motion of shaft 400 tolinear movement of compensating block 410 and finally to arcuatemovement of the tip 430 of the pivot arm to which the bracket 354 andassociated retroreflector 350 are attached (See FIG. 5). This providesfor the tuning of the output beam of the laser.

The rotary portion of the drive train includes: shaft 400, rim 404,rotary flex member 406 and cylindrical nut 414. In the embodiment shown,the actuator 370 is a rotary actuator and specifically a stepper motor.As will be obvious to those skilled in the art, suitable alternateactuators include: piezo-electric stacks, AC/DC motors, linear steppermotors, etc. The output shaft 400 of the stepper motor is coupled viathe hub and rim 402-404 to the rotary flex member 406, which is in turncoupled to the cylindrical nut 414. The cylindrical nut includes athreaded interior portion. The rotary flex member 406 is placedintermediate the cylindrical nut and the drive shaft 400 in order tode-couple the cylindrical nut from any misalignments of the steppermotor shaft 400. These misalignments can arise, for example, due tonon-parallelism between the axes of the lead-screw assembly and motor,or run-out and wobble in the motor-shaft, nut and screw. The rotary flexmember is relatively compliant in all directions except longitudinally.The torsional compliance of the driveshaft introduces hysteresis intothe system. This is overcome by driving the motor to approach all targetpositions from the same direction. In this way the “wind-up” of thedriveshaft becomes a constant, rather than a variable. The rim 404passes through the start condition optical switch 392 and is encoded(See FIG. 9), so as to allow the switch to sense an arcuate startingpoint for the actuator shaft. After registering that starting locationduring the initialization of the signal generator, no further detectionis required for the switch(s).

The linear portion of the drive train includes translation unit 412,compensating element 410, and lead screw 418. The lead screw 418includes a threaded portion which engages the interior threaded portionof the cylindrical nut. The head of the cylindrical nut is coupled tothe distal end of the compensating element 410. The compensating element410 is in turn coupled to the linear translation unit 412. The lineartranslation unit 412 is coupled to the motor bracket 310. Thus, rotationof the stepper motor shaft 400 results in a linear movement of the leadscrew 418 toward, or away from, the cylindrical nut with which it isthreadably engaged. The movement of the lead screw is linearized withrespect to the base by means of the attachment of the nut to the basevia the compensating element 410 and translation unit 412. As will beobvious to those skilled in the art, the placement of the lead screw andnut could be reversed without departing from the scope of the invention.In that alternate embodiment of the invention, the rotary member wouldhave an external thread, i.e. lead screw, and the cylindrical nut wouldbe attached to the compensating element. In still another embodiment ofthe invention, the linearization of the lead screw and compensatingelement could be achieved by the positioning of the head of the leadscrew within a complementary opening of the base, thereby linearizingthe motion of the lead screw with respect to the base. In an alternateembodiment of the invention the lead screw is rotationally driven andaxially constrained as shown in FIGS. 13B-C.

The arcuate portion of the drive train includes the linear flex member420, fasteners 422-424, and the tip 430 of the pivot arm. In theembodiment shown, the flex member 420 is a spring metal strip, thecross-sectional profile of which is rectangular. In alternateembodiments of the invention, the linear flex member may include squareor round, cross-sectional profiles. The linear flex member allowsconversion of the linear motion of the compensating element into anarcuate motion of the tip 430. In an alternate embodiment, the linearflex member comprises part of the tip 430 of the pivot arm.

FIG. 5 is an exploded isometric view of the tunable laser shown in FIGS.3-4, in which the actuator and drive train assembly have been omitted.The relationship of the primary components of the tunable laser to acommon base or ground plane is shown. The laser diode housing 330couples to mounting holes 504 within base 300 via fasteners 500-502. Thediffraction grating 340 couples to the mount 342. This coupling may beby means of an adhesive fastener, soldered, welded or integral with thebase. The mount 342 couples to mounting holes 510 within base 300 viafasteners 506-508. The pivot member 550 is rotatably coupled to the base300 at thru-hole 532, the center of which is aligned with the pivot axis530. In a preferred embodiment of the invention, the pivot axis locationwith respect to the laser diode, diffraction grating and retroflector isdetermined in accordance with the teachings embodied in the '668 Patent.Significantly, the pivot point location takes into account the effect ofthe dispersion of the laser and other optical elements in the system onthe cavity length. This pivot point is selected so as to provide aninternal cavity length (See FIGS. 11-12) which is substantially aconstant integer number of half-wavelengths throughout all wavelengthswithin the tuning range. Bearing post 540 is fit into the thru-hole fromthe bottom side of the base 300. The base portion 552 of the pivotmember 550 includes a cylindrical bearing 560. The bearing is fit overthe post on the bottom of the base 300, thereby providing preciserotation of the pivot member in a plane parallel to the lower surface ofthe base plate 300. Attached to an intermediate portion 554 of the pivotmember is the above-mentioned pivot bracket 354. This extends from thebottom of the base to an exposed position on the top side of the base.The pivot member 550 is secured to the lower portion of the base 300 viamounting plate 570 and fastening members (not shown). In the assembledposition (See FIG. 6), the compensating element 352 and retroreflector350 are coupled to the pivot bracket from the top side of the base 300.The fiber coupling 304 and fiber mount 302 are fastened to the base viafasteners (not shown) positioned within mounting holes 520 definedwithin the base.

An advantage to the embodiment of the tunable laser shown in FIGS. 3-5is that all components are coupled to a common base. Consistent with theteachings on the current invention, the locations of each of thecomponents can be precisely calculated. Thus, as is the case in theprior art, it is not necessary that adjustment features be provided forany of the components. Instead, the laser diode, diffraction grating,and retroreflector are either absolutely or rotatably fixed to a commonbase, thereby greatly improving the stability of the output signalsgenerated by the tunable laser. In alternate embodiments of theinvention, the apparatus for coupling the pivot member 550 to the baseincludes: rotary bearing, needle bearing, journal bearings, flexuralbearings, rotary flexural bearings, etc.

FIG. 6 is an assembly isometric view of the tunable laser shown in FIG.5 in which the actuator and drive train assembly have been omitted. Thelaser diode housing 330, diffraction grating 340, and fiber coupling 304are shown coupled to the base 300. The pivot bracket 354 extendspartially above the top surface of base 300. The proximal end of thecompensating element 352 is attached to the pivot bracket by fasteners(not referenced). The retroreflector 350 is coupled to the distal end ofthe compensating element. In the embodiment shown the retroreflector isfastened by means of an adhesive, solder, weld, etc. Finally, the tip430 of the pivot member 550 is shown beneath the top of the base andextends into the upper portion of the base in which the drive trainassembly will be located.

FIG. 7 is an exploded isometric view of the drive train portion of thetunable laser shown in FIGS. 3-4. Specifically, the rotary and linearportions of the drive train are shown with the arcuate portion omitted.The actuator 370, motor bracket 310, start condition detectors 390-392,and drive train assembly 376 are shown. In the exploded view, the lineartranslator 412 is shown with a lower portion 740 coupled to the base ofthe motor bracket 310 in an orientation which provides for linearmovement along the longitudinal axis defined by the drive assembly ofthe linear translator. This axis, as will be discussed in the followingFIG. 13, is generally tangent to the arc swept out by the tip of thepivot arm. The start condition detectors 390-392 are shown coupled tothe motor bracket. The strap 440 and mounting holes 312 provide twofastening points by which the drive assembly and actuator are rigidlycoupled to the base 300. In an alternate embodiment of the invention,the actuator and translation unit may be directly coupled to the base300. The rotary portion of the drive assembly 376 includes the actuatorshaft (not shown), hub 402, rim 404, rotary flex member 406 andcylindrical nut 414. A notch 720 on the rim 404 is shown. Whenassembled, the rim rotates within opposing arms of the rotary startcondition sensor 392, that notch is optically detected, therebyaccurately gauging an arcuate starting position of the actuator 370.

The lead screw 418 threadably couples at a proximal end to the interiorthreaded portion of the cylindrical nut 414. Thus, as the cylindricalnut is rotated by the actuator, the lead screw is retracted or extendedwithin the cylindrical nut. The lead screw is affixed at a distal end toa distal end 710 of the compensating element 410. The attachment of thecompensating element to a translation unit 412 both linearizes themovement of the compensating element, as well as prevents the rotationof the lead screw. This limits the lead screw and compensating elementto the desired linear motion along the longitudinal drive axis.

The linear flex member 420 is a strip of spring metal generallyrectangular in cross-section and with lower and upper portions 700-702,respectively. At a proximal end, the lower portion is attached viafasteners 422 to the compensating element 410. The point of attachmentis precisely determined at a distance from the distal end 710 of thecompensating element. At a distal end the flex member is coupled viafasteners 424 to the tip 430 (not shown) of the pivot member 550 (SeeFIG. 5). The thermal expansion of the compensating element is calculatedso as to thermally pacify the drive assembly and prevent steady statemotion of the pivot arm 550 and retroreflector (See FIG. 13) as a resultof temperature variations.

FIG. 8 is an assembled view of the drive assembly 376, motor bracket 310and actuator 370 shown in FIG. 7. The drive assembly is shown attachedto the actuator. The rim 404 is positioned within start conditiondetector 392 to detect rotational orientation of the actuator. Thecompensating element 410 is fastened to the linear translator 412, whichis in turn coupled to the motor bracket 310. The compensating element isconstrained to linear motion with respect to the base along a linetangent to the arc swept by the tip of the pivot arm 550 (See FIG. 5)during tuning of the laser. The upper portion 702 of the linear flexmember 420 is positioned within start condition sensor 390 such that thestart position of the compensating element may be detected.

FIG. 9 is an assembly view of the embodiment of the tunable laserdiscussed above in connection with FIGS. 3-8. The base 300, actuator 370and motor bracket 310 are not shown. The laser diode housing 330 ispositioned above the base 552 of the pivot arm. The pivot bracket,located at an intermediate portion of the pivot arm, is shown with theretroreflector 350 attached thereto via the compensating element 352.The retroreflector 350 is caused to undergo a combination of rotationand translation with respect to the diffraction grating 340 (shown inphantom view) by means of arcuate motion of the tip 430 of the pivotarm. The distal portion of the lead screw 418 (See FIG. 4) andcompensating element 410 have been removed in order to view the lowerportion 700 of the flex member. The lower portion of the linear flexmember is coupled via fasteners 424 to the tip 430 of the pivot arm. Theupper portion 702 of the linear flex member is shown positioned withinthe linear start condition detector 390. Rotation of the drive shaft ofthe actuator results in rotation of the cylindrical nut 410. Thisresults in linear movement of the lead screw and the compensatingelement to which it is attached. This linear motion in turn arcuatelydisplaces the pivot arm through the coupling of the tip of that arm tothe compensating element via the linear flex member. The linear flexmember is sufficiently rigid so as to overcome any friction in the pivotarm bearing 560 (See FIG. 5), thus assuring that for each unique lineardisplacement of the compensating element, a unique pivot arm angle isalso defined. This retroreflector coupled to the pivot arm is therebycaused to undergo both rotation and translation with respect to thediffraction grating.

In operation, a laser diode within housing 330 emits a beam 900 throughthe front facet which intersects at a grazing angle the diffractiongrating 340. The diffracted beam 902 from the grating strikes theretroreflector 350. A portion of the diffracted beam having a specificwavelength determined by the orientation of the retroreflector withrespect to the grating is reflected back to the grating and injectedback into the laser diode, thus selecting a cavity mode that supportsthe desired output wavelength. The reflection 904 of the laser beam fromthe diffraction grating provides a potential source for the opticaloutput 260 (See FIG. 2) of the signal generator of which the tunablelaser is a part. An alternate output signal source is provided by beam906 from the back facet of the laser. This optional beam results whenthe back facet of the laser diode is partially transmissive.

Output Wavelength Determination

In order for the tunable laser to be controlled with an open loopsystem, which does not require closed loop feedback of, for example,wavelength or position, several requirements must be met in embodimentsof the invention in which the laser is mechanically tuned. First, theactuator which drives the tuning element must be capable ofincrementally moving the tuning element, e.g. retroreflector,diffraction grating, etalon, etc., from one position to the next acrossthe tuning range so that narrowly separated center wavelengths can beselected. Second, there must be some way of correlatingcontrol/activation signals supplied to the actuator with outputwavelength. Third, in the absence of wavelength or position feedback,there must be some means of maintaining the correlation betweencontrol/activation signals and the output wavelength, even in thepresence of environmental variations. Temperature variations, forexample, cause the drive train, base, pivot arm, and other componentswithin the tunable laser to expand/contract, thereby varying the outputwavelength.

The first of these requirements is fulfilled by the combination of arotary actuator, such as a stepper motor, with the de-amplificationprovided by a cylindrical nut and a finely pitched lead screw. The pitchof the lead screw determines the amount of linear movement produced thatwill resolve from each rotation of the stepper motor. As finer pitchedthread is utilized on the lead screw 418, the wavelength resolution ofthe system will increase. In an alternate embodiment of the invention,the wavelength resolution may be increased by increasing the length ofthe pivot arm.

The second requirement is fulfilled by the combination of the startcondition sensors 390-392, the actuator 370, and the lookup table 212.Start conditions sensors may be used to determine a base location forone or more of: the tuning element; the pivot arm; the arcuate, linearor rotary portions of the drive train; or the actuator. In theembodiment shown, the start condition sensors each have a small cavitywith a beam of light emitted from one side which is detected on theother side. Interrupting the beam changes the state of the sensor. Whenthe processor 206 (See FIG. 2) initializes the system, the actuator iscaused to turn until the upper portion 702 of the flex member eitherinterrupts or clears the light beam of linear sensor 390. If the systemexhibits hysteresis, then the direction in which the actuator makes afinal approach at the starting point will be the same each time, thusremoving the effect of hysteresis. The linear sensor may be positionedon any part of the tuning system, e.g. the drive assembly, pivot arm,tuning element, etc.

Where the accuracy of the linear start condition sensor alone isinsufficient to indicate a unique starting condition, the rotary startcondition sensor 392 may be used in combination with the linear sensor.Unlike the linear sensor, the rotary sensor does not have a unique startcondition where the actuator output shaft makes more than one rotationacross the tuning range. Thus, when used in combination, the linear androtary sensors operate sequentially, with the linear sensor required togive a first indication of a start condition, and the rotary sensorproviding a subsequent indication. In this embodiment, the processoractuates the stepper motor in a pre-defined direction, i.e. clockwise orcounterclockwise, until the linear sensor is triggered. Subsequently,the stepper motor is backed off in the reverse direction, and thenenergized in the forward direction until the rotary sensor 392 changesstate. The predefined direction for triggering the change of state ofsensor 392 assures that backlash/hysteresis is removed from the driveassembly. Sensors other than linear or rotary may be used to signal thestart condition. In an alternate embodiment of the invention, the startcondition sensor(s) may be electrically coupled to the actuator to sensean overload current/voltage level thereof. When the actuator moves thedrive train to a mechanical endpoint, the increase in the drivevoltage/current level resulting from the increased load on the actuatorcould be used to signal the start condition. Alternately, responsive toa unique output wavelength, an inexpensive optical sensor could be usedto signal the start condition. In still another embodiment of theinvention, microswitches, capacitative sensors inductive sensors,magnetic read switches, etc. could be utilized to signal the startcondition.

Once the base condition has been indicated, no further signaling fromeither the linear/rotary or other start condition sensor is requiredduring the selection of output wavelengths for the device. Instead, anopen loop control system is utilized in which the processor using thelookup table determines the type/quantity of drive signals relative tothe base state that are required to move the tuning element to theselected output wavelength and drives the actuator accordingly. Theactuator accepts drive signals, and responsive thereto producesincremental movements, e.g. arcuate displacements from the base state.Where a high degree of accuracy is required, the lookup table is uniqueto each device. The processes associated with generating the lookuptable are set forth in FIGS. 10 and 18. The processes for generatingselected output wavelengths are set forth in the following FIG. 19.Although satisfaction of both the first and second requirements is anecessary condition for implementing an open loop control system for thesignal generator, alone or in combination, they are not a sufficientcondition where high degrees of wavelength accuracy and resolution arerequired. The signal generator must be environmentally stable as well.

One of the primary factors affecting both accuracy and repeatability ofthe combined drive unit and laser is temperature. Small changes in theangle of the tuning element, induced not by the actuator but by thermalexpansion, can vary the output wavelength from one to another of thenarrowly separated output wavelengths. Thus, a signal generator withoutfeedback of position or wavelength may not exhibit a unique/repeatableoutput wavelength in response to a given drive signal sequence unlessthe signal generator is thermally stable. FIGS. 11A-B and FIG. 12 showembodiments of the invention for thermally stabilizing optical pathlength. FIG. 13 shows an embodiment of the invention for thermallystabilizing a mechanically actuated tuning element of an external cavitylaser.

Generating a Lookup Table

FIG. 10 shows an embodiment of the invention for generating a lookuptable. The tunable laser discussed above is superimposed on themultimeter hardware layout shown in FIG. 2. An input of a wavelengthmeter 1000 is shown connected to the output beam 260 from the signalgenerator 250. The output from the signal generator is coupled throughthe I/O interface to CPU 206 and memory 208. During the assembly of eachsignal generator or groups thereof, the signal generator is hooked up toan external multimeter in a final stage of the assembly process. Next,the processor 206, using program code 210 in memory 208, energizes theactuator 230 and monitors the start condition detectors 240 until astart condition is indicated. Then, the wavelength measured by thewavelength meter is recorded as the first/base record in thedatabase/lookup table 212. Next, the processor sends a knowncombination/amount/type of activation signals to the actuator 230 whichresults in the tuning of the laser to a next wavelength level. Thecombination/amount/type of activation signals is recorded along with thewavelength measured by the wavelength meter in the database/lookup table212 as the next record therein. The process is repeated to generatesubsequent records. Next, additional records may be generated in thelookup table/database by interpolation between existing records. Whenthe population of the lookup table is complete, the table isdownloaded/stored in the memory 208 of the multimeter.

In an alternate embodiment of the invention, the lookup table isgenerated using an external processor and memory in combination with theexternal wavelength meter. The lookup table is generated in a mannersubstantially similar to that discussed above. The processor drives theactuator; the wavelength meter indicates the output wavelength of theoutput beam 260. The processor records the correlation betweenwavelength and actuator drive signals and stores the results in thelookup table. Then, after the signal generator is assembled into theoptical multimeter, the lookup table for the signal generator portion ofthe multimeter is downloaded to the memory 208. Further details on theprocesses associated with generating the lookup table/database 212 areset forth in FIG. 18.

Thermally Stabilizing the Optical Path Length

Temperature changes affect the overall cavity length and index ofrefraction of the cavity, which in turn result in variations in outputwavelength as well as mode hops. As the optical length of the lasercavity varies with respect to temperature, the integral number ofhalf-wavelengths that may be supported in the cavity varies. The opticalpath length of a cavity is a function of the physical thickness of eachelement, optics and air included in the cavity, and the refractive indexof the element. Two elements with identical thickness and differentindices of refraction will each support a different number ofhalf-wavelengths along their thickness since the speed of light variesinversely with refractive index. Thus an element with a higherrefractive index, e.g. glass, supports a greater number of wavelengthsover an identical physical length than an element, such as air, with alower refractive index.

Once an output wavelength is selected, any variations in the opticalpath length in the cavity result in discontinuities, a.k.a. “mode hops”,in the output beam brought about by variations in the integral number ofhalf-wavelengths in the cavity. These variations may be brought about bya combination of physical path length variations and/or variations inthe indices of refraction of the elements within the cavity, including:optics, gain medium, and any gas such as air.

FIGS. 11A-D show alternate embodiments of a tunable laser with acompensating element for passively stabilizing the optical path lengthof a laser cavity during variations in temperature. FIGS. 11A-C areelevation views of variations on the Littman-Metcalf configuration. FIG.11D is an elevation view showing the Littrow configuration. Eachincorporate compensating elements. The compensating element(s) work byexpanding/contracting along the optical axis by an amount sufficient tooffset any temperature related contraction/expansion in the optical pathlength, to thermally stabilize the optical path length. In FIGS. 11A-B atunable external cavity diode laser with fixed proximal and distal endsand an intermediate tuning element is shown. In FIGS. 11A-B, acompensating element attaches an optical component to the base of thelaser in a manner which respectively decreases and increases the opticalpath length during expansion of the compensating element. In FIG. 11C anexternal cavity diode laser with a fixed tuning element, e.g.diffraction grating, and a variably positioned proximal and/or distalend(s) is shown with a compensating element which decreases the opticalpath length during expansion. In FIG. 11D an external cavity diode laserwith a fixed gain medium and a variably positioned tuning element isshown with a compensating element which decreases the optical pathlength during expansion.

The tunable laser of FIG. 11A includes: foundation 1100, gain medium1120, optical elements 1128, tuning element 1130 and a retroreflector1126. The optical elements, tuning element and retroreflector provide aretroreflective tuning device which tunes the laser by providingfeedback of a selected wavelength to the gain medium. In an embodimentof the invention, the gain medium is a laser diode with front and rearfacets 1124-1122, respectively. In various embodiments of the invention,the optical elements 1128 include lenses and filters. In variousembodiments of the invention, the tuning element 1130 includes aninterference filter, an Etalon, a diffraction element, and a grating. Inthese embodiments, tuning is accomplished by rotation and/or translationof the tuning element. In other embodiments of the invention, the tuningelement includes an optical crystal the wavelengthabsorption/transmission of which varies with an applied current orvoltage. In various embodiments of the invention, the retroreflectorincludes a mirror, a comer cube and a dihedral prism. A resonant cavityis formed with a length L_(Opl) between the rear facet 1122 of the laserdiode 1120 and the retroreflector 1126. The resonant cavity includes aninternal cavity between the rear and front facets 1122-1124 of the laserdiode and an external cavity between the front facet 1124 of the laserdiode and the retroreflector 1126.

At the proximal end, the laser diode 1120 is fixed to the foundation1100 at pad 1102. At the distal end of the cavity, the retroreflector isfastened to a compensating element 1118. At one end, the compensatingelement is coupled to the base 1100 at pad 1104. At the opposing end,the compensating element fastens to the retroreflector. Pad 1104 ispositioned outside the optical path, beyond the retroreflector. Thus, asthe compensating element expands, the retroreflector is pushed into thecavity reducing the length of the cavity. As the temperature of thefoundation increases, the separation between pads 1102-1104 changes,typically for most materials, increasing as well. The compensatingelement 1118 offsets this physical expansion of the base by expanding inan amount which maintains a constant optical path length L_(opl). Aswill be obvious to those skilled in the art, the compensating elementmay be positioned elsewhere in the cavity, for example, joining the gainmedium to the base, without departing from the scope of the invention.In still another embodiment of the invention, there may be more than onecompensating element positioned between, for example, theretroreflector-base and gain medium-base connections.

The compensating element should be designed to maintain an opticalpathlength which does not vary with temperature. Satisfaction of thisrequirement assures that instances of thermally induced mode hopping orvariations in output wavelength will be substantially reduced. As shownin FIG. 11A, the optical pathlength L_(Opl) may be expressed as the sumof the optical paths through the individual components of the tunablelaser including: the diode 1124, the optical element(s) 1128, the tuningelement 1130 and the air gaps La₁,La₂,La₃ between the various elements.The optical path length through the diode is L_(d). The optical pathlength through the optical element(s) is L₁. The optical path lengththrough the tuning element is L_(t). The optical path length through theair gap between the laser and optical element(s) is La₁. The opticalpath length through the air gap between the optical and tuningelement(s) is La₂. The optical path length through the air gap betweenthe tuning element and the retroreflector is La₃. Since all elements aredirectly or indirectly coupled in a fixed or pivoting manner to the base1100, their relative physical separation will typically increase as thetemperature of the base increases. This may in turn vary the opticalpathlength of the cavity.

The optical pathlength of an element is equal to the product of itsrefractive index and its dimension along the optical path. The opticalpathlength of the cavity of the tunable laser is the sum of the productsof index of refraction and thickness along the optical path for allelements, including air, within the cavity. This requirement isexpressed in the following Equation I, in which n_(i) is the index ofrefraction of each element and 1 _(i) the physical thickness of theelement along the optical path.

Equation I

L _(Opl) =Σn _(i) ·l _(i)

The lower case “l” will reference the physical dimension of an elementand the upper case “L” the optical dimension. The integer number ofhalf-wavelengths supported by an element with fixed endpoints increasesas the refractive index of the element increases, as predicted byHuygens principle. This results from the observation that light travelsslower in media of higher index of refraction, and the wave peaks aremore closely packed. Thus, over an identical distance, an element with ahigher index of refraction supports a greater number of wavelengths.Thus, the optical path length rather than physical pathlength is a moreaccurate measure of the integral number of half-wavelengths which acavity can support.

Nevertheless, as a first order approximation, the thermal expansionrequired by the compensating element 1118 is that required to maintainthe physical pathlength dimension of the cavity, i.e. l_(Opl) constant.That requirement would be met provided d1 _(F1)/dT=d1 _(C)/dT for theconfiguration shown in FIG. 11A. Given the physical distance betweenattachment points 1102-1104 and the coefficient of thermal expansionα_(F) of the base 1100, the required combination of material andthickness between pad 1104 and retroreflector 1126 could be determinedso as to hold the physical distance between the cavity endpoints1122,1126 constant. There would several sources of error in the firstorder approximation. First, optical and physical pathlength are notequivalent as discussed above. Instead, for each segment of the opticalpath, e.g. L_(d), L_(l), L_(t), L_(a1),−L_(a3), the refractive index ofeach element must be considered in order to hold the integer number ofhalf-wavelengths in the cavity constant. Second, in determining thenumber of wavelengths each element can support, the expansion of theelement must be calculated. Expansion of each element varies dependingon its coefficient of thermal expansion and cross-sectional thicknessalong the optical path. Additionally, during temperature variations,some cavity elements may expand while others contract, thus varying theaverage weighted refractive index of the cavity. The average weightedrefractive index being the sum of the products of the physical lengthand refractive index of each element divided by the physical length ofthe cavity. For example, during a temperature increase, the air gapL_(a3) may decrease due to the rapid inward expansion of thecompensating element while the optical element(s) increase in thickness.Thus the average weighted refractive index may vary as a result. A thirdsource of error results from the fact that the refractive index of eachelement varies with temperature and by different amounts. What is neededis a way of incorporating all these variables into the choice ofmaterial and thickness for the compensating element(s) so that thecavity is optically stable over a broad temperature range.

A more accurate way of determining the combination of material andthickness for the compensating element(s) 1118 is provided in thefollowing Equation II in which the temperature related variation inoptical path length both due to changes in the physical length of eachelement as well as the change in the index of refraction of each elementis expressed.

Equation II$0 = {\frac{L_{Opl}}{T} = {{\sum\frac{\left( {n_{i} \cdot l_{i}} \right)}{T}} = {\sum{\left( {{n_{i} \cdot \alpha_{i}} + \frac{n_{i}}{T}} \right) \cdot l_{i}}}}}$

In Equation II, the requirement that the rate of change of the opticalpathlength L_(Opl) with respect to temperature be zero satisfies thecondition that the optical pathlength be temperature invariant. Theoptical path length is expressed as the sum of the derivatives of theproduct of the refractive index “n_(i)” of each element, the thermalexpansion coefficient “α_(i)” of each element and the physical length“l_(i)” of each element. As stated above, the elements of the cavityinclude: laser, optics, filters, and gasses, such as air, in the opticalpath.

The optical path of the laser shown in FIG. 11A is the sum of theoptical length of the individual segments of which it is composedincluding the columns of air/gas separating the elements. Thisrelationship is expressed in the following solution EI-1a to the abovementioned Equation I.

Solution EI-1a

L _(Opl) =L _(d) +L _(l) +L _(t) +L _(a123) =n _(d) l _(d) +n _(l) l_(l) +n _(t) l _(t) +n _(a) l _(a123)

Now the last term, i.e. the air gap length l_(a1-3), is affected byexpansion and contraction of the base 1100 as well as the compensatingelement 1118. The air gap length can be expressed in terms of thedimensions of the base l_(F1) and compensating element l_(c). Theappropriate substitutions have been made in the following SolutionEI-1b.

Solution EI-1b

L _(Opl) =n _(d) l _(d) +n _(l) l _(l) +n _(t) l _(t) +n _(a)(l _(F1) −l_(d) l _(l) −l _(t) −l _(c))

Next the terms are rearranged in Solution EI-1c to express the opticalpath length in terms of: L_(F1) the optical length of the base, L_(O)the additional optical length produced by the optical elements in thecavity, and L_(C) the optical length of the compensating element(s).

Solution EI-1c and Equivalent Expression $\begin{matrix}{L_{Opl} = \quad {{+ \left\lbrack {n_{a}l_{F1}} \right\rbrack} + \left\lbrack {{\left( {n_{d} - n_{a}} \right)l_{d}} + {\left( {n_{1} - n_{a}} \right)l_{1}} + {\left( {n_{t} - n_{a}} \right)l_{t}}} \right\rbrack - \left\lbrack {n_{a}l_{c}} \right\rbrack}} \\{{\quad \quad^{''}} = \quad {L_{F} + L_{O} - L_{C}}}\end{matrix}$

Then the derivative of L_(Opl) is found and set equal to zero, asindicated in Equation II. This provides a solution for the derivative ofthe optical length of the compensating element(s) L_(C)′ in terms of thesum of the derivative L_(F)′ of the optical length of the base andL_(O)′ the additional optical length produced by the optical elements inthe cavity as set forth in the following Solution EII-1d. Thecoefficients of thermal expansion α_(c), α_(F), α_(d), α_(l), for: thecompensating element, the base, the gain medium, e.g. laser diode, thelens, and the tuning element, respectively will be utilized in solvingthe derivative. In addition, the indices of refraction n_(a), n_(d),n_(l), and n_(t) for air, the diode, the optical elements and the tuningelement will be utilized in solving the following derivative.

Solution EII-1d and Equivalent Expression $\begin{matrix}{\left\lbrack {n_{a}l_{c}} \right\rbrack^{\prime} = \quad {{+ \left\lbrack {n_{a}l_{F}} \right\rbrack^{\prime}} + \left\lbrack {{\left( {n_{d} - n_{a}} \right)l_{d}} + {\left( {n_{1} - n_{a}} \right)l_{1}} + {\left( {n_{t} - n_{a}} \right)l_{t}}} \right\rbrack^{\prime}}} \\{L_{C}^{\prime} = \quad {L_{F}^{\prime} + L_{O}^{\prime}}}\end{matrix}$

The derivatives in solution EII-1d may be solved for to produce asolution for the product of the coefficient of thermal expansion andlength of the compensating element(s).

FIG. 11B shows a different compensating block to base geometry than thatof FIG. 11A. In FIG. 11B, the compensating element attaches the opticalcomponent to the base of the laser in the opposite manner to thatdiscussed above. Expansion of the compensating element 1118 in FIG. 11Bincreases the optical path length during expansion of the compensatingelement. As in FIG. 11A, the optical path of the laser shown in FIG. 11Bis the sum of the optical length of the individual segments of which itis composed. This relationship is expressed in the following solutionEI-2a to the above mentioned Equation I.

Solution EI-2a

L _(Opl) =L _(d) +L _(l) +L _(t) +L _(a124) =n _(d) l _(d) +n _(l) l_(l) +n _(t) l _(t) +n _(a) l _(a124)

As before, the air gap length l_(a1-3) is affected by expansion andcontraction of the base 1100 as well as the compensating element 1118,however, in this case the expansion of the compensating element has theopposite effect. The air gap length can be expressed in terms of thedimensions of the base l_(F1) and compensating element l_(c). Theappropriate substitutions have been made in the following SolutionEI-2b. Only the sign of the last term has changed from that of SolutionEI-1b to reflect the fact that the optical element expansion adds to thecavity length.

Solution EI-2b

L _(Opl) =n _(d) l _(d) +n _(l) l _(l) +n _(t) l _(t) +n _(a)(l _(F1) −l_(d) −l _(l) −l _(t) +l _(c))

Next, the terms are rearranged in Solution EI-2c to express the opticalpath length in terms of: L_(F1) the optical length of the base, L_(O)the additional optical length produced by the optical elements in thecavity, and L_(C) the optical length of the compensating element(s).

Solution EI-2c and Equivalent Expression $\begin{matrix}{L_{Opl} = \quad {{+ \left\lbrack {n_{a}l_{F1}} \right\rbrack} + \left\lbrack {{\left( {n_{d} - n_{a}} \right)l_{d}} + {\left( {n_{1} - n_{a}} \right)l_{1}} + {\left( {n_{t} - n_{a}} \right)l_{t}}} \right\rbrack - \left\lbrack {n_{a}l_{c}} \right\rbrack}} \\{{\quad \quad^{''}} = \quad {L_{F} + L_{O} - L_{C}}}\end{matrix}$

Then, the derivative of L_(Opl) is found and set equal to zero, asindicated in Equation II. This provides a solution for the derivative ofthe optical length of the compensating element(s) L_(C)′ in terms of thesum of the derivative L_(F)′ of the optical length of the base, andL_(O)′ the additional optical length produced by the optical elements inthe cavity as set forth in the following Solution EII-2d. Thecoefficients of thermal expansion α_(c), α_(F), α_(d), α_(l), for: thecompensating element, the base, the gain medium, e.g. laser diode, thelens, and the tuning element, respectively will be utilized in solvingthe derivative. In addition, the indices of refraction n_(a), n_(d),n_(l), and n_(t) for air, the diode, the optical elements and the tuningelement will be utilized in solving the following derivative.

Solution EII-2d and Equivalent Expression $\begin{matrix}{{- \left\lbrack {n_{a}l_{c}} \right\rbrack^{\prime}} = \quad {{+ \left\lbrack {n_{a}l_{F}} \right\rbrack^{\prime}} + \left\lbrack {{\left( {n_{d} - n_{a}} \right)l_{d}} + {\left( {n_{1} - n_{a}} \right)l_{1}} + {\left( {n_{t} - n_{a}} \right)l_{t}}} \right\rbrack^{\prime}}} \\{{- L_{C}^{\prime}} = \quad {L_{F}^{\prime} + L_{O}^{\prime}}}\end{matrix}$

This in turn may be solved to produce a solution for the product of thecoefficient of thermal expansion and length of the compensatingelement(s).

The tunable laser of FIG. 11C also includes: foundation 1100, gainmedium 1120, optical elements 1128, tuning element 1130 and aretroreflector 1126. The optical elements, tuning element andretroreflector provide a retroreflective tuning device which tunes thelaser by providing feedback of a selected wavelength to the gain medium.In an embodiment of the invention, the gain medium is a laser diode withfront and rear facets 1124-1122, respectively. In various embodiments ofthe invention, the optical elements 1128 include lenses and filters. Invarious embodiments of the invention the tuning element includes aninterference filter, an Etalon, a diffraction element, and a grating. Aresonant cavity is formed with a length L_(Op1) between the rear facet1122 of the laser diode 1120 and the retroreflector 1126. The resonantcavity includes an internal cavity between the rear and front facets1122-1124 of the laser diode and an external cavity between the frontfacet 1124 of the laser diode and the retroreflector 1126.

In various embodiments of the invention, the retroreflector includes amirror, a corner cube and a dihedral prism. In these embodiments, tuningmay be accomplished by rotation/translation of the retroreflector 1126which is pivotally fastened to the base at pivot point 1112 viacompensating element 1118 and pivot arm 1110. In alternate embodimentsof the invention, the tuning may be accomplished by rotation/translationof the gain medium with respect to the base.

As the temperature of the foundation increases, the separation betweenpads 1102-1104 changes, typically for most materials, increasing aswell. The compensating element 1118 offsets this physical expansion ofthe base by expanding in an amount which maintains a constant opticalpath length L_(opl). As will be obvious to those skilled in the art, thecompensating element may be positioned elsewhere in the cavity, forexample joining the gain medium to the base, without departing from thescope of the invention. In still another embodiment of the inventionthere may be more than one compensating element positioned between, forexample, the retroreflector-base and gain medium-base connections.

FIG. 11D shows a Littrow configuration of an external cavity diode laserwith a fixed gain medium and a variably positioned tuning element, e.g.a diffraction grating 1150. The optical elements 1128 and tuning element1150 provide a retroreflective tuning device which tunes the laser byproviding feedback of a selected wavelength to the gain medium. Tuningis accomplished by rotation/translation of the tuning element, e.g.grating 1150 which forms the distal end of the cavity. The grating ispivotally fastened to the base at pivot point 1112 via compensatingelement 1118 and pivot arm 1110. In alternate embodiments of theinvention, the tuning may be accomplished by rotation/translation of thegain medium with respect to the base. A resonant cavity is formed with alength L_(Opl) between the rear facet 1122 of the laser diode 1120 andthe tuning element 1150. The resonant cavity includes an internal cavitybetween the rear and front facets 1122-1124 of the laser diode, and anexternal cavity between the front facet 1124 of the laser diode and thetuning element 1150.

As the temperature of the foundation increases, the separation betweenpads 1102-1104 changes, typically for most materials, increasing aswell. The compensating element 1118 offsets this physical expansion ofthe base by expanding in an amount which maintains a constant opticalpath length L_(opl). As will be obvious to those skilled in the art, thecompensating element may be positioned elsewhere in the cavity, forexample joining the gain medium to the base, without departing from thescope of the invention. In still another embodiment of the inventionthere may be more than one compensating element positioned between, forexample, the retroreflector-base and gain medium-base connections.

FIG. 12 is a top plan view of the resonant cavity portion of the tunablelaser signal generator 250 (See FIG. 2). The laser is tuned by aretroreflective tuning device which tunes the laser by providingfeedback of a selected wavelength to the gain medium, e.g. laser diode332. The tuning device includes diffraction grating 340 andretroreflector 350. The relative physical location of the lasercomponents is affected by the expansion of the base and further byexpansion of any intermediate elements, e.g. housings, or mountingblocks, which may be used to fasten the laser components to the base.Laser components in FIG. 12 include: laser diode 334, diffractor 340,retroreflector 350, as well as any lens or filters that may be present.Housing 330, diffraction mount 342 and compensating element 352 areintermediate elements used to fasten the corresponding laser componentto the base. A resonant cavity is formed with a length L_(opl1)+L_(opl1)between the rear facet 334 of the laser diode 332 and the tuning element350. The resonant cavity includes an internal cavity between the rearand front facets 1122-1124 of the laser diode and an external cavitybetween the front facet of the laser diode and the tuning element 350.

Absent intermediate members, the relative physical separation betweenoptical components will increase with temperature since all componentsare attached in a fixed or pivotal manner to a common base which expandswith an increase in temperature. Intermediate members may be used toeither increase or decrease the relative physical separation betweenoptical components during a temperature-induced expansion of the base.In the embodiment shown, all intermediate members, i.e. housing 330,diffraction mount 342 and compensating element 352, make contact withthe base at locations outside the optical path. Laser diode 332 iscoupled via housing 330 to the base. The housing contacts the base atcontact line 1200, which is displaced outside the optical path bydistance l_(Cd). The laser housing is fastened to the base by fastenersalong a centerline 1204. Thus, expansion of the housing reduces thelength of air gap l_(a1), between the front facet of the laser diode andthe diffraction grating. Diffraction grating 342 is coupled via mount340 to the base. For purposes of simplifying the solution set thatfollows, it is assumed that the diffraction mount 342 and base haveidentical coefficients of expansion and that the expansion coefficientof the diffraction grating is zero. In this specific case theintermediate component, i.e. mount 342, does not create relativeexpansion/contraction of the diffraction grating surface with respect tothe base. Were this not the case, the solution set that follows wouldtake into account the reduction in length of both optical path segmentsL_(Opl1) and L_(Opl2) resulting from differential expansion of thesurface of the diffraction grating and the base. Retroreflector 350 iscoupled via compensation element 352 to the pivot bracket 354, which isin turn pivotally coupled to the base 300. The compensation elementcontacts the pivot bracket at contact line 1202, which is displacedoutside the optical path by distance l_(Cr). Thus, expansion of thecompensation element reduces the length of air gap l_(a2) between thefront face of the retroreflector and the diffraction grating.

As will be obvious to those skilled in the art, intermediate members maybe fabricated in different lengths of different materials, with varyingcoefficients of expansion less than, or greater than that of the base.If they have higher coefficients of thermal expansion than the base towhich they are attached, then their expansion tends to decrease thephysical separation between components and may be used to counteract orcompletely offset expansion of the base. Conversely, were theintermediate components rearranged to make contact with the base atlocations within the optical path, they would have the opposite effect,i.e. increasing the relative separation between optical componentsbeyond what would be the case, were the optical components attacheddirectly to the base. Thus, one or more intermediate members may be usedwith a base and laser components to thermally induce separations betweenoptical components which either vary directly/inversely withtemperature. This capability will be relied on to fabricate a thermallystable signal source.

As discussed above in Equation II, the requirement of a thermally stableoptical pathlength is met when the rate of change of the opticalpathlength L_(Opl) with respect to temperature is zero. In theembodiment shown in FIG. 12, the optical path of the laser is folded toinclude two distinct segments L_(Opl1) and L_(Opl2), between the laserdiode 332 together with the diffraction grating 340, and the diffractiongrating together with retroreflector 350, respectively. The totaloptical pathlength L_(Opl12) is the sum of the optical length of alloptical components within each of the segments including the columns ofair/gas separating the elements. This relationship is expressed in thefollowing solution EI-3a to the above-mentioned Equation I.

Solution EI-3a

L _(OPl12) =L _(d) +L _(r) +L _(a12) =n _(d) l _(d) +n _(r) l _(r) +n_(a) l _(a12)

Now the last term, i.e. the air gap length l_(a12) is affected byexpansion and contraction of the base 300 as well as the compensatingelement 352 and housing 330. The air gap length can be expressed interms of the dimensions of the base l_(F1-2), compensating elementl_(Cr) and diode housing l_(Cd). The appropriate substitutions have beenmade in the following Solution EI-3b.

Solution EI-3b

L _(Opl12) =n _(d) l _(d) +n _(r) l _(r) +n _(a)(l _(F1) −l _(Cd) −l_(d) +L _(F2) −l _(Cr))

Next the terms are rearranged in Solution EI-3c to express the opticalpath length in terms of: L_(F12) the optical length of the base, L_(O)the additional optical length produced by the optical elements in thecavity, and L_(C) the optical length of the compensating element(s).

Solution EI-3c and Equivalent Expression $\begin{matrix}{L_{Opl12} = \quad {{+ \left\lbrack {n_{a}\left( {l_{F1} + l_{F2}} \right)} \right\rbrack} + \left\lbrack {{\left( {n_{d} - n_{a}} \right)l_{d}} + {n_{r}l_{r}}} \right\rbrack - \left\lbrack {n_{a}\left( {l_{Cd} + l_{Cr}} \right)} \right\rbrack}} \\{{\quad \quad^{''}} = \quad {L_{F} + L_{O} - L_{C}}}\end{matrix}$

Then the derivative of L_(Opl12) is found and set equal to zero, asindicated in Equation II. This provides a solution for the derivative ofthe optical length of the compensating element(s) L_(C)′ in terms of thesum of the derivative L_(F)′ of the optical length of the base andL_(O)′ the additional optical length produced by the optical elements inthe cavity as set forth in the following Solution EII-3d. Thecoefficients of thermal expansion α_(Cd), α_(Cr), α_(F12), α_(d),α_(r)for: the laser housing, compensating element, base, diode,retroreflector respectively will be utilized in solving the derivative.In addition, the indices of refraction n_(a), n_(d), and n_(t) for air,the diode, and the tuning element will be utilized in solving thefollowing derivative. Additionally, where a collimating lens ispositioned at the output of the laser diode the index of refraction andthermal expansion coefficient for that element would appear as well inthe following equation.

Solution EII-3d and Equivalent Expression $\begin{matrix}{\left\lbrack {n_{a}\left( {l_{Cd} + l_{Cr}} \right)} \right\rbrack^{\prime} = \quad {{+ \left\lbrack {n_{a}l_{F12}} \right\rbrack^{\prime}} + \left\lbrack {{\left( {n_{d} - n_{a}} \right)l_{d}} + {n_{r}l_{r}}} \right\rbrack^{\prime}}} \\{L_{C}^{\prime} = \quad {L_{F}^{\prime} + L_{O}^{\prime}}}\end{matrix}$

This in turn may be solved to produce a solution for the product of thecoefficient of thermal expansion and length of the compensatingelement(s).

In an alternate embodiment of the invention the length of thecompensating elements can be obtained experimentally by measuring thewavelength of the composite cavity and using this information todetermine the length of the compensating element(s).

Thermally Stabilizing the Drive Train

Thermal variations in a mechanically tuned laser affect not only theoptical pathlength but also the angle of the tuning element. Bothoptical pathlength variations, as well as changes in the tuning angle,contribute to thermally induced mode hop and wavelength variations inthe output beam. Typically, thermal stabilization of the opticalpathlength as discussed above, is a necessary but not sufficientcondition for reducing thermally induced mode hop and wavelengthvariations in the output beam. FIGS. 13B-D show embodiments of theinvention for passive thermal stabilization of the angle of a tuningelement in a mechanically tuned laser. FIG. 13A shows a prior art designin which variations in the tuning angle may be thermally induced.

FIG. 13A shows a prior art design for a mechanical drive train to movethe tuning element of an external cavity laser. A base 1300, pivot arm1302, tuning element 1310, lead screw 1340 and threaded block 1320 areshown. The pivot arm is fastened to the base 1300 at pivot point 1304.The tuning element 1310, e.g. retroreflector, grating, etalon, etc., isattached to the pivot arm such that arcuate movement of the pivot arm,induced by the lead screw 1340, tunes the laser (not shown). The leadscrew is flexibly attached to the tip of the pivot arm 1342. The leadscrew has an elongated threaded portion extending from the tip of thepivot arm through a threaded opening in the threaded block to a driveend 1344 of the lead screw. The threaded block is fixed to the base1300. As the lead screw is rotated by an actuator (not shown), it moveslinearly along a line tangent to the tip of the pivot arm. The arcuatemotion of the pivot arm induced, thereby tunes the laser by rotating thetuning element to a specific angle with respect to the base. This inturn selects a specific output wavelength for the laser. At any selectedoutput wavelength, the angle must be held constant during temperaturevariations in order to avoid variations in the output wavelength. In theprior art case shown in FIG. 13A, this requirement is met only when thethermal expansion coefficient α_(Dt) of the lead screw 1340 and baseα_(B) are identical. In the unique case where this condition is met, theexpansion of the drive train, e.g. lead screw, along length D_(Dt) willequal that of the base D_(B) over the distance separating the tip of thepivot arm from the centerline of the threaded block 1320. In a practicalimplementation, this condition will typically not be met since the baseis typically fabricated from a very hard, thermally inert material suchas a nickel-steel alloy, and the lead screw of a soft, easily machinedmaterial with a relatively high coefficient of expansion, such as brass.Therefore, in the typical case, the prior art drive train design is notthermally stable since the differential expansion of the drive trainexceeds that of the base. Thus, prior art tunable lasers are subject totemperature induced tuning of the laser, i.e. “thermal tuning”, whichcreates undesirable variations in the output wavelength of the laserand/or mode hopping. Therefore, what is needed is a way to use materialssuitable for the drive train and base without the requirement that theyhave identical expansion coefficients.

FIGS. 13B-D show various embodiments of the invention for thermallystabilizing the drive train. A compensating element is provided tooffset the differential expansion between the base and the driveassembly. In the embodiments shown, the compensating element is linkedto the drive train in a geometry which offsets the differentialexpansion thereby enhancing the thermal stability of the tuning elementat any selected output wavelength.

FIG. 13B shows a drive train similar to that discussed above inconnection with FIG. 13A, with the exception of compensating element1322 which couples the drive train to the base. The compensating elementis U-shaped with a threaded opening in the base and with a rim which isaffixed to the base. The compensating element is laid out on its sidewith the lead screw passing through the opening in the rim and through athreaded opening in the base of the compensating element to a point oftermination at the drive end 1344 of the lead screw. The compensatingelement typically has an overall thermal expansion greater than that ofthe base 1300 by an amount sufficient to compensate for differentialexpansion of the base and lead screw. That relationship is expressed inthe following Equation III, where d_(B) is the length of the base fromthe tip of the pivot arm to the fastening point for the rim 1324, d_(c)is the length of the compensating element, and d_(dt) is the lead screwlength from the tip of the pivot arm to the base of the compensatingmember. The thermal expansion coefficients for the compensating element,base, lead screw are: α_(c), α_(dt), α_(b) respectively.

Equation III

+α_(c) d _(c) =α _(dt) d _(dt) −α _(b) d _(b)

FIG. 13C shows an alternate embodiment of the compensating element forthermally stabilizing the mechanical drive train. In this embodiment,the lead screw is stationary and is rotatably fastened at opposite endsto the base via pillow blocks 1330 and 1328 on either side of pivot arm1304. Shoulders on the lead screw on either side of pillow block 1328maintain a fixed relationship between the lead screw and that pillowblock. Expansion of the lead screw exhibits itself at pillow block 1330in which the lead screw is free to move linearly. Movement of the tip ofthe pivot arm results from the threaded attachment at an intermediatepoint on the lead screw of a threaded portion of the base ofcompensating element 1350, with the rim of that element attaching to thetip 1342 of the pivot arm. As the lead screw rotates in a clockwise orcounterclockwise direction, the threaded base of the compensatingelement is caused to undergo linear translation along a line tangent tothe tip. This movement produces arcuate movement of the tip to tune thelaser. The compensating member 1350 offsets the differential expansionbetween the drive train assembly, e.g. lead screw 1340, and the base byexpanding in a direction opposite to the expansion of the lead screw soas to maintain the pivot arm in a fixed position. That relationship isset forth in Equation III above.

As will be obvious to those skilled in the art, the thermalstabilization provided by the compensating element is equally applicableto laser drive trains such as: piezo-electric actuators, solenoids,linear stepper motors, etc., without departing from the scope of theinvention.

FIG. 13D is a top plan view of the embodiment of the tunable laserdiscussed above in connection with FIGS. 3-9. The base 300, drive train376, retroreflector 350, diffraction grating 340, pivot arm tip 430 andmotor attachment bracket 310 are shown. The hole 532 about which thepivot arm rotates is shown. In the embodiment shown, the drive shaftincluding stepper motor output shaft, rotary flex member, cylindricalnut, and lead screw, has a length of d_(Dt). The compensation element410 is coupled to the end of the drive shaft to the head of the leadscrew. As the drive shaft expands along length d_(Dt), the compensatingelement expands in the opposite direction over the length d_(c) to thepoint at which one end of the linear flex member 420 is coupled to thecompensating element. The compensation element will typically have acoefficient of expansion greater than either the drive train or thelinear flex member. It is dimensioned such that its expansion offsetsthe difference between the expansion of the drive shaft together withthe linear flex member from that of the base. In the embodiment shown,for purposes of simplification, the faceplate of the stepper motor atwhich the drive shaft originates is assumed to be fixed to the base atlocation 1380. The base expands over the distance d_(B) measured fromthe tip 430 of the pivot arm to the origin of the actuator drive shaft.These parameters are set forth in accordance with Equation III in thefollowing solution EIII-1a.

Solution EIII-1a

+α _(c) d _(c)=(α_(dt) d _(dt)+α_(b) d _(b))−α_(b) d _(b)

In this embodiment of the invention, passive thermal compensation of thedrive train achieves the effect of maintaining a stable angle betweenthe tuning elements, i.e. retroreflector 350 and the diffraction grating340. This assures that the output wavelength will remain temperatureinvariant on any output channel/frequency. In combination, passivethermal pathlength compensation and thermal compensation of the drivetrain also substantially reduce mode hopping. As will be obvious tothose skilled in the art, it is evident that the roles of the lead screwand cylindrical nut may be reversed without departing from the scope ofthe invention. In an alternate embodiment of the invention, the lengthof the compensating element(s) can be obtained experimentally bymeasuring the wavelength of the composite cavity and using thisinformation to determine the length of the compensating element(s).

Accurate Positioning of Components

Thermal path length compensation requires accurate positioning of thelaser components. In addition to accurate positioning, the line/point ofcontact between each component of the laser system, as well as anyintermediate elements necessary to fasten them to the base, must bedetermined. In order to properly dimension compensating elements, suchas the laser housing, it is preferable that they frictionally contactthe base along a narrow and well-defined line of contact. From this lineof contact, expansion and contraction calculations necessary fordetermining the length and material combination for the intermediatecompensating components may be calculated.

FIGS. 14A-B show respectively an isometric exploded view and a sidecross-sectional view of pads which are used, in an embodiment of theinvention, to position the laser components with respect to one. Thesepads improve the precision of the relative thermal expansioncalculations necessary to dimension the laser components andintermediate elements properly so as to thermally stabilize the opticalpath length (See FIGS. 11A-D and 12). They do so by reducing the contactarea between the attached objects, e.g. a laser component orintermediate element and the base. Additionally the pads serve toprovide three points of contact or contact along a line together with apoint of contact to level the device as well as accurately position it.Typically two or more pads will be utilized between the attachedobjects. Where two pads are utilized, the first, a contact pad, willtypically provide a narrow line of contact from which expansioncalculations are performed while the second, a leveling pad, provides alow friction surface area to level the attached component orintermediate element. The line of contact provided by the contact padwill typically be orthogonal to the optical path. The contact pad willtypically have a triangular or narrow rectangular cross-section toincrease frictional contact between it and the objects between which itis sandwiched. The leveling pad will typically have a broad rectangularcross-section with a smooth surface to allow the objects on either sideto move relative to one another during thermal expansion/contraction.Fasteners between the contact and leveling pad will be utilized toapportion the loading on each. Typically, the greater loading will beplaced on the contact pad to increase the friction between it and theobjects between which it is sandwiched. A reduced loading on theleveling pad allows relative movement between the objects on eitherside. The pads may be separate from the corresponding attached object orpart of either of them. In the absence of these contact pads, thermalexpansion calculations would be made from the centerlines of thefasteners used to couple laser components or intermediate elements tothe base. This latter technique may lack the precision provided by thecontact pads due, for example, to the slop between fasteners and thethru and threaded holes of the attached objects.

In FIG. 14A, a three pad fastening system is shown for the attachment ofthe laser housing 330 to the base 300. There are two contact pads1400-1402 and one leveling pad 1404. Within the base are defined thefastening holes 504, 510 and 520 for fastening respectively the laserhousing 330, diffraction mount 342 and fiber mount 302 to the base (SeeFIGS. 3-5). Pads 1400-1404 are positioned between the base 300 and thelaser housing 330. The two contact pads 1400-1402 are aligned with oneanother along contact line 1200 which is generally orthogonal to theoptical path. These two pads provide the frictional contact with thehousing from which thermal path length calibration will be calculated.The remaining leveling pad 1404 is laid out on axis 1410 and serves tolevel the housing and has a light enough contact with the housing sothat the housing is slidably positioned with respect to this pad. Thelaser housing 330 is brought into contact with the pads by fasteners500-502 which threadably engage holes 504 having a centerline 1204within the base 300.

FIG. 14B shows a cross-sectional side elevation view of the base 300 andlaser housing 330. The housing is shown contacting both contact pad 1402and leveling pad 1404. Fasteners 500 are shown positioned at a distanced₁ from the contact pad 1402, and a distance d₂ from the leveling pad1404. The contact force between the laser housing and the contact pad isF×d₂/(d₁+d₂) where “F” is the fastening force. As d₁ decreases, theforce on the leveling pad, i.e. F×d₁/(d₁+d₂), decreases as well. Theseparation l_(cd) between the contact pad 1402 and the rear facet 334 ofthe laser diode 332 (See FIG. 3.) is selected in combination with thethermal expansion coefficient of the laser housing material to provide,in combination with the other components and compensating elements ofthe tunable laser, a thermally stable optical pathlength as discussedabove in connection with FIGS. 11A-B and 12. In alternate embodiments ofthe invention the contact pads may be integral with either of theelements being fastened, or may be fastened between them. The contactpads may have varying cross sectional profiles with the contact pad(s)typically having a narrow high friction profile to prevent relativemovement of the objects being fastened. The leveling pad by contrast hastypically a planar surface to minimize friction and allow relativeexpansion between the objects fastened.

Active Thermal Compensation

In an embodiment of the invention, active thermal stabilization may beutilized alone or in combination with the passive techniques discussedabove to maintain wavelength stability and avoid mode hopping. Activethermal compensation avoids temperature related wavelength variationsand mode hopping by maintaining the tunable laser components at aconstant temperature. By actively adding or removing energy from thecavity responsive to feedback from temperature/energy monitors, arelatively constant thermal state can be maintained for the tunablelaser. This approach requires heaters/coolers as well as closed loopfeedback sensors and circuitry. In an alternate embodiment of theinvention a less expensive approach to active thermal stabilization maybe implemented. In this approach there is no active feedback, relyinginstead on maintaining a temperature in the tunable laser that issignificantly above or below the ambient condition so as to reduceexternal environmental effects on the laser. To avoid additionalcomponents such as heaters/refrigerators, it is advantageous to utilizethe existing components in the system where possible to provide therequisite energy input. The actuator holds such potential. In a steppermotor, for example, energy is consumed in moving the tuning element fromone to another output wavelength. By designing the stepper motor controlto output a constant power level at any pole, or phantom pole, and evenin a locked condition, the overall thermal variations in the tunablelaser may be kept at a relatively constant temperature.

FIG. 15 shows a detailed circuit diagram of an embodiment of themodulation circuit 222 discussed above in connection with FIG. 2. Thecircuit provides, as is shown in the following FIG. 16, a range ofanalog and digital modulation which is suitable for testing the variousoptical components associated with an optic network (See FIG. 1). Thecircuit provides a relatively low-frequency feedback loop formaintaining a stable output power that operates in combination with arelatively high frequency open loop switched threshold voltage source1510 and a laser power shunt to inject a digital modulation signal ontoa selected peak output power of the optical signal generator. Themodulating circuit includes a setpoint module to generate a fixed outputcurrent/voltage, a first modulation module 1510 to switchably connectthe output of the setpoint module to an input of the feedback module1520; and a second modulation module 1580 to switchably connect thelaser diode/gain medium 1584 to a current source 1566 and a controlunit. The laser diode/gain medium 1584 is part of the tunable laser,e.g. gain medium 224 in FIG. 2 or laser diode 332 in FIG. 4.

In the embodiment shown, the set point module comprises an analog todigital converter 1502 and a voltage controlled current source 1504. Theanalog to digital converter 1502 drives the voltage controlled currentsource 1504 to a specific output current/voltage which is provided as aninput to the first modulation module 1510.

The first modulation module 1510 includes a transistor switch 1512, pulldown resistor 1514, and resistor bridge 1516-1518. In a first position,switch 1512 couples the input from the set point module to the pull downresistor 514, which is in turn coupled to ground. In a second position,the switch 1512 couples the output of the set point module to the nodeformed between resistors 1516-1518. This raises or lowers the voltage atthe node of the resistor bridge. The resistor bridge is coupled at oneend to a ground within the first set point module 1510, and at thealternate end provides an input to the low frequency feedback module1520.

The low frequency feedback module accepts inputs from the laser diode1584 and the first modulation module 1510, and provides an output to theanalog modulator 1562 and the voltage controlled current source 1566 towhich it is attached. The feedback module includes: a beam splitter1544, a photodetector 1542, a summer, and an integrator. In theembodiment shown, the summer is an op amp 1524 with a bridging resistorbetween the negative input and output. The positive output of the op ampis coupled via resistor 1522 to ground. The negative input of the op ampcouples both to the output resistor 1518, which is part of the first setpoint module 1510, as well as to the photodetector 1542 via intermediateresistor 1540. Thus, at the negative input, the op amp sums the currentprovided by the first modulation module with the current withdrawn bythe photodetector 1542. The output of the summer is coupled via resistor1528 to the input of the integrator. The integrator includes: op amp1532, bridging capacitor 1534, and resistor 1536, which couple betweenthe negative input of the op amp and the output. The positive input ofthe op amp is coupled via resistor 1530 to ground. At the output, theintegrator couples via analog modulator 1562 to the voltage controlledcurrent source 1566. Within the feedback loop, the beam splitter 1544accepts as an input the output beam 1546 provided by laser diode 1584.This beam is split into an output portion 1548 and a feedback portion1550. The feedback portion 1550 drives the photodetector 1542. When thesystem is in equilibrium, the amount of current withdrawn by the photodiode 1542 would be equivalent to the current provided by the firstmodulation module 1510 at the negative input of the op amp 1524. In thissteady state condition, the amount of current provided by current source1566 will be that required to drive the laser diode 1584 at a powerlevel determined by the output level of the set point module 1500. Anyvariations in the set point module will result in more or less currentprovided by the current detector 1566.

The laser diode 1584 couples to the second modulation module 1580. Thesecond modulation module includes a switch 1582 and a pull up resistor1586. The switch switchably couples either the laser diode 1584 or thepull up resistor 1586 to the current source 1566, which is in turnconnected via resistor 1564 to ground. The current detector 1568monitors the current through laser diode 1584.

The control module provides a control input to the set point module 1500and specifically the analog to digital converter 1502 therein. Thecontrol module also provides control inputs to both switches 1512 and1582 in, respectively, the first and second modulation modules.Additionally, the control module provides an input to analog modulator1562. The control module accepts input from the current detector 1568.The control module 1560 is in turn coupled via system bus 216 to theprocessor 206 (See FIG. 2).

In operation, the user selects an output channel/wavelength for theoptical signal generator which is tuned to that wavelength via theactuator and drive train assembly, lookup table and processor asdiscussed above and in the following FIG. 19. Next, a specific powerlevel, digital modulation frequency, and duty cycle are selected.Responsive to the power selection, the control module 1560 generates asignal to the analog to digital converter 1502 within the set pointmodule which results in the appropriate current being delivered bycurrent source 1504 to the first modulation module 1510. Then,responsive to the user-selected modulation frequency and duty cycle, thecontrol unit 1560 generates signals which cause switches 1512 and 1582within the first and second modulation modules to switch between polesat a rate and duty cycle proportional to the inputs from the controlmodule. Switches 1512-1582 are operated substantially synchronously suchthat in the first position switch 1512 shunts the output of the setpointmodule via resistor 1514 to ground and switch 1582 in the first positioncouples the pull up resistor 1586 to the current source 1566. Thus, inthe first position, no input is provided from the first modulationmodule to the feedback unit 1520, and no current is delivered to thelaser 1584. In the second position, switch 1512 couples the output ofthe set point module to the resistor bridge 1516-1518 which providescontrol input to the feedback module 1520, and specifically the summerthereof. In the second position, switch 1582 couples the laser diode1584 to the current source 1566. By virtue of the substantiallysynchronous operation of switches 1512 and 1582, the relatively lowfrequency feedback circuit 1520 is not required to engage in digitalmodulation, seeking instead a relatively constant peak output state thatcan be maintained across any range of duty cycles and modulationfrequencies which can be implemented by switches 1512-1582.

An additional feature of the modulation circuit 222 is that analogmodulation capability is provided. At a constant DC power level orduring digital modulation, control module 1560 can provide an analoginput to analog modulator 1562. In an embodiment of the invention,analog modulator 1562 is a pull down resistor which adds or removescurrent from the line connecting the output of the integrator to thevoltage controlled current source. An additional feature of themodulation circuit is the provision of overload current protectionprovided by detector 1568. Detector 1568 provides a signal proportionalto the current through the laser diode 1584. The signal is provided tothe control module 1560 which, in conjunction with the processor 206 towhich it is coupled, causes the switch 1582 to de-couple the laser diode1584 from the current source 1566 when an overload condition isdetected.

As will be obvious to those skilled in the art the modulator may beimplemented using either analog or digital circuits or software, singlyor in combination without departing from the scope of the invention. Inone digital embodiment of the circuit an integrator within the errordetector/feedback circuit 1520 would integrate only in the on state whenthe laser diode was coupled to the current source.

The modulating circuit shown in FIG. 15 may be utilized with equaladvantage in numerous lasers including: distributed feedback lasers,YAGG lasers, gas lasers, tunable semiconductor lasers, distributed Braggreflectors, etc., without departing from the scope of the invention. Infact, the modulating circuit may be utilized in any laser in whichmodulation of output beam intensity can be accomplished.

FIG. 16 is a graph showing some of the various optical output signalprofiles which can be generated by the optical signal generator 250 (SeeFIG. 2) under the control of the modulation circuit 222 (See FIGS. 2,15). The laser output beam may be modulated across a range of dutycycles, frequencies and power levels. Signal sequences 1600-1604 areshown. In signal sequence 1600, the set point module 1500 (See FIG. 15)provides a fixed output current/voltage while the first and secondmodulation module alternately de-couple and couple the set point moduleand laser diode from the feedback circuit 1520 across a range of dutycycles at a fixed frequency and a power level P₁. In signal sequence1602, the set point module 1500 (See FIG. 15) provides a second powerlevel P₂ while the first and second modulation modules couple andde-couple the laser diode and set point module with the feedback circuitacross a range of frequencies at a fixed duty cycle. In signal sequence1604, both the frequency and the duty cycle of the first and secondmodulation modules is fixed, and the set point module 1500 delivers avoltage/current sufficient to drive the current source 1566 (See FIG.15) at a third power level P₃. In signal sequence 1608, a fixed dutycycle and frequency is provided by the control module to the first andsecond modulation modules 1510-1580 (See FIG. 15), while the set pointmodule 1500 is ramped from the first to the third power level. Enlargedsignal diagram 1620 shows a portion of signal sequence 1604 in which theanalog modulator 1562 under the control of the controller 1560 (See FIG.15) injects an analog signal onto the output beam 1548 by modulating thecurrent source 1566. This analog sequence is injected only on thepositive going digital modulation sequence since only during thatportion of the signal sequence is the laser diode 1584 coupled to thecurrent source 1566.

FIG. 17 shows an embodiment of the data structure associated with thelookup table 212 utilized during open loop operation of the signalgenerator 250 (See FIG. 2). During open loop operation, the processor206 (See FIG. 2) responds to the user selection of a specific outputwavelength by implementing processes (See FIG. 19) which in conjunctionwith the lookup table result in the appropriate drive signals beingdelivered to the actuator (See FIG. 2) so as to cause the laser to emitan output beam at the selected wavelength. Database 212 comprises anumber of wavelength records, each of which contains a wavelength field1704 and a drive signal/pulse field 1702. A first of the wavelengthrecords, i.e. the base record, additionally contains a flag 1700indicating that it is the starting point for further calculations. In anembodiment of the invention, this flag would be the beginning of file(BOF) or end of file (EOF) indicator or a specific starting addresswithin the database in which the lookup table 212 was contained. In theembodiment shown, the first record has entries of “0” for a pulse countand a wavelength of 1525 nm. The second record has a pulse count of 4and a wavelength of 1525.5 nm. The third record has a pulse count of 8and a wavelength of 1526 nm. In the embodiment shown, the pulses aretotal cumulative pulses required to move the actuator from the basewavelength to the wavelength associated with the cumulative number ofpulses. The following process flow FIG. 18 shows the processesassociated with generating the lookup table.

FIG. 18 is a process flow diagram showing the processes associated withgenerating the lookup table 212 (See FIG. 10). Processing begins atstart block 1800, in which the system for driving the signal generator250 and for measuring the output wavelength from the wavelength meter110 and storing that wavelength in a lookup table, are initiated.Control then passes to process 1802. In process 1802 the actuator 230 isgradually swept from a starting position until, in the followingdecision step 1804, a signal is received from a first start conditionsensor indicating that the base state has been reached. In an embodimentof the invention, that sensor, e.g. sensor 390 and/or 392 (See FIGS.3-9) indicates that the start/base position for the pivot arm has beenreached. Control then passes to process 1820.

In an embodiment of the invention which implements a combined linear androtational sensor such as that shown in FIG. 9, control may alternatelypass from decision process 1804 to processes 1806-1812 for a base statedetermination by a second sensor. In process 1806, any backlash isremoved from the drive system by sending appropriate activating pulsesto the actuator. Control is then passed to process 1810 in which theactuator is energized. Control then passes to decision process 1812 inwhich a determination is made as to when a second sensor, e.g. sensor392 (See FIG. 3) indicates that the base position has been reached. Whenthis determination is made, control is passed to process 1820.

In process 1820, the measurement of the output wavelength at the baseposition is obtained from the wavelength meter 1000 (See FIG. 10).Control is then passed to process 1822. In process 1822, the wavelengthmeasurement is stored in the first record in the database along with thedrive signal sequence/amount associated with the base position, e.g. apulse count of “0”. Control is then passed to process 1824. In process1824 the processor 206 (See FIG. 10) or its equivalent sends a fixedsequence/type/number of activation signals to the actuator 230 whichresults in the tuning of the laser to a next wavelength level. Controlis then passed to process 1826. In process 1826 the pulses generated inprocess 1824 are added to the previous amount to generate a cumulativepulse count. Control is then passed to process 1828. In process 1828 thewavelength measurement made by the wavelength meter 1000 (See FIG. 10)is obtained. Control is then passed to process 1830. In process 1830 thewavelength obtained in process 1828 and the cumulative pulse countobtained in process 1826 are combined into a single record which isstored in a database 212 (See FIG. 2). Control is then passed todecision process 1832. In decision process 1832 a determination is madeas to whether the last wavelength obtained in process 1828 lies at theend of the operating range of the signal generator. In the event thatdetermination is in the negative, control returns to process 1824 forthe next increment of the actuator. Alternately, if in decision process1832 an affirmative is reached, that the signal generator has reachedthe end of the operating range, control is then passed to decisionprocess 1834. In decision process 1834 a determination is made as towhether additional records will be generated by interpolation. If thatdetermination is negative, then control is passed to process 1838. Ifthe determination is affirmative, control passes to process 1836. Inprocess 1836 an interpolation is performed using existing records in thedatabase, and additional records corresponding to interpolations betweenthe initial records in the database are added to the database. Theseadditional interpolated records each have a pulse count offset from thebase and a wavelength. Then control passes to process 1838. In process1838 the completed database with records correlating pulse count andwavelength is stored in memory 208 (See FIGS. 2-10). In an alternateembodiment of the invention multiple traces, averages, curve fitting maybe used to generate additional records. In still another embodiment ofthe invention, measurements of drive signals and output wavelengths maybe made across the tuning range to establish a functional relationshipbetween wavelength and drive signals. In this embodiment, the lookuptable would contain the single function correlating wavelength withdrive signals rather than a plurality of records.

FIG. 19 is a process flow diagram showing the processes associated withoperation of the signal generator portion of the multimeter 100.

Processing begins at start block 1900 in which the signal generator isinitialized. Control then proceeds to process 1902. In process 1902 theCPU 206 (See FIG. 10) outputs drive pulses to the stepper motor causingit to initiate a slow sweep from a start position. Control is thenpassed to decision process 1904. In decision process 1904 adetermination is made as to whether a start condition sensor hassignaled the base position. In the event that determination is in theaffirmative, control may optionally be passed to additional processes1906-1912 for the confirmation of a second sensor as to the basecondition. Alternately, control is passed directly to process 1920.

In optional process 1906 any backlash in the drive train is removed andcontrol is passed to process 1910. In process 1910 the processor outputsdrive signals to the stepper motor. Control is then passed to decision1912. In decision process 1912, a determination is made as to when asignal is received from the second sensor, e.g. sensor 392, indicating abase position. Control is then passed to process 1920.

In process 1920 the wavelength and pulse count for the base position areread from the lookup table 212 (See FIGS. 2, 10). These are stored inthe history register. Control is then passed to process 1922. In process1922 the wavelength value read from the base record in the lookup tablemay be displayed on display 200 (See FIG. 2). Control is then passed todecision process 1924. In decision process 1924 a determination is madeas to when the next output channel/center wavelength has been selected.Selection may result from a number of input sources. These sourcesinclude entries from the user via user inputs 202 (See FIG. 2) or fromprogram code stored in memory 208 and having a specific operating regimefor the signal generator. In either event, once a determination is madethat the next wavelength/channel has been indicated, control is thenpassed to process 1926. In process 1926 a lookup is performed on thelookup table/database using the wavelength obtained in decision process1924. If the next wavelength corresponds to that of a wavelength recordin the database, then that record including the associated cumulativepulse count is read by the processor 206. Alternately, if the targetwavelength does not match any of the records of the database, then thetwo closest records in the database are obtained and an interpolation ofthe pulse count stored in each is performed to generate a cumulativepulse count or drive signal profile which lies in between the tworecords. Control is then passed to process 1928. In process 1928, thepulse count stored in the history register in process 1920 is subtractedfrom the pulse count obtained in process 1926. Control is then passed todecision process 1930. In decision process 1930 a determination is madeas to whether the difference obtained in decision process 1928 has apositive or negative value. If the value is positive, indicating thatmovement of the actuator in the same direction is appropriate to achievethe next output wavelength, then control is passed directly to process1940. Alternately, if the difference obtained is negative, control ispassed to intermediate process 1932. In intermediate process 1932appropriate pulses are output, e.g. amounting to the difference obtainedin process 1928 plus an additional backlash value. Control is thenpassed to process 1934 in which the backlash is reversed. Control isthen passed directly to process 1944 in which the wavelength obtained indecision process 1924 is displayed to the user.

Alternately, if in decision process 1930 a determination is made thatthe difference is positive then control is passed directly to process1940. A positive determination as discussed above indicates that thereis no backlash/hysteresis to remove since the movement to the nextwavelength selected is in the same direction as was utilized in theprevious measurement. In process 1940 the pulse difference obtained inprocess 1928 is output by the processor to the actuator. Control is thenpassed to decision process 1944 in which the desired wavelength isdisplayed to the user. Then control returns to decision process 1924 forthe processing of the next selected output wavelength.

The many features and advantages of the present invention are apparentfrom the written description, and thus, it is intended by the appendedclaims to cover all such features and advantages of the invention.Further, since numerous modifications and changes will readily occur tothose skilled in the art, it is not desired to limit the invention tothe exact construction and operation as illustrated and described.Hence, all suitable modifications and equivalents may be resorted to asfalling within the scope of the invention.

What is claimed is:
 1. A tunable laser, comprising: a base; a gainmedium coupled to the base; a tunable feedback device coupled to saidbase to provide feedback of a selected wavelength to the gain medium;and a first compensating element providing the coupling to said base forone of said gain medium and said tunable feedback device such thatthermal expansion of said first compensating element maintains asubstantially constant integer number of half-wavelengths within aresonant cavity defined by the gain medium and the tunable feedbackdevice during temperature variations in the tunable laser.
 2. Thetunable laser of claim 1, further comprising: a diffraction element; anda tuning element coupled to said diffraction element to adjust anorientation of said diffraction element with respect to said gain mediumto vary the selected wavelength.
 3. The tunable laser of claim 1,further comprising: a diffraction element having dispersion planes; aretroreflector within the dispersion planes; and a tuning element foradjusting an orientation of at least one of said diffraction element andsaid retroreflector to vary the selected wavelength.
 4. The tunablelaser of claim 1, further comprising: an interference element; aretroreflector; and a tuning element for adjusting an orientation ofsaid interference element to vary the selected wavelength.
 5. Thetunable laser of claim 1, further comprising: a second compensatingelement providing the coupling to said base for an other of said gainmedium and said tunable feedback device such that thermal expansion ofsaid first compensating element and said second compensating elementmaintains a substantially constant integer number of half-wavelengthswithin a resonant cavity defined by the gain medium and the tunablefeedback device during temperature variations in the tunable laser. 6.The tunable laser of claim 1, further comprising: a contact pad betweensaid first compensating element and said base; a leveling pad betweensaid first compensating element and said base; and a fastener forfastening said first compensating element together with one of said gainmedium and said tunable feedback device to said base, and the fastenerpositioned between said contact pad and said leveling pad to apportion afastening force to permit relative movement between said leveling padand at least one of said base and said first compensating element and tosubstantially reduce relative movement of said base and said contact padthereby providing at said contact pad a single location from which todetermine the required thermal expansion characteristics of said firstcompensating element.
 7. The tunable laser of claim 6, wherein saidcontact pad provides one of: two points of contact between said base andsaid first compensating element, and a line of contact between said baseand said first compensating element.
 8. The tunable laser of claim 6,wherein the line of contact provided by said contact pad between saidbase and said first compensating element lies substantially orthogonalto an optical path within the resonant cavity.
 9. The tunable laser ofclaim 6, wherein one of said base and said first compensating elementdefine at least one of said contact pad and said leveling pad integralwith a respective surface portion thereof.
 10. A tunable laser,comprising: a base; a gain medium coupled to the base; a first feedbackdevice coupled to the base to provide feedback of a selected wavelengthto said gain medium; a pivot arm with a proximal and a distal end, andthe proximal end of said pivot arm pivotally attached to the base at afirst pivot axis; a second feedback device coupled to the distal end ofsaid pivot arm to provide feedback of the selected wavelength to saidfirst feedback device, and said second feedback device together withsaid first feedback device and said gain medium defining a resonantcavity, and the second feedback device responsive to the arcuatedisplacement of said pivot arm to vary the selected wavelength; and afirst compensating element coupling the second feedback device to thedistal end of said pivot arm such that thermal expansion of said firstcompensating element maintains a substantially constant integer numberof half-wavelengths within the resonant cavity.
 11. The tunable laser ofclaim 10, wherein the positioning of the first pivot axis provides bothrotation and translation of the second feedback device with respect tothe first feedback device during the arcuate displacement of said pivotarm to vary the selected wavelength together with maintaining asubstantially constant integer number of half-wavelengths in theresonant cavity.
 12. The tunable laser of claim 10, wherein the firstfeedback element includes a diffraction element and the second feedbackdevice includes a retroreflector.
 13. The tunable laser of claim 10,wherein the first feedback element includes a retroreflector and thesecond feedback device includes an interference element.
 14. The tunablelaser of claim 10, further comprising: a second compensating elementcoupling said gain medium to said base such that thermal expansion ofsaid first compensating element and said second compensating elementmaintains a substantially constant integer number of half-wavelengthswithin the resonant cavity.
 15. The tunable laser of claim 14, furthercomprising: a contact pad between said second compensating element andsaid base; a leveling pad between said second compensating element andsaid base; and a fastener for fastening said second compensating elementtogether with said gain medium to said base, and the fastener positionedbetween said contact pad and said leveling pad to apportion a fasteningforce to permit relative movement between said leveling pad and at leastone of said base and said second compensating element and tosubstantially reduce relative movement of said base and said contact padthereby providing at said contact pad a single location from which todetermine the required thermal expansion characteristics of said secondcompensating element.
 16. The tunable laser of claim 15, wherein thecontact pad provides one of: two points of contact between said base andsaid second compensating element, and a line of contact between saidbase and said second compensating element.
 17. The tunable laser ofclaim 16, wherein the line of contact provided by said contact padbetween said base and said second compensating element liessubstantially orthogonal to an optical path within the resonant cavity.18. The tunable laser of claim 15, wherein one of said base and saidsecond compensating element define at least one of said contact pad andsaid leveling pad integral with a respective surface portion thereof.19. A laser apparatus comprising: (a) first and second optical elementspositioned to define a resonant optical cavity; and (b) a compensatingelement associated with at least one of said optical elements, saidcompensating element configured such that thermal expansion of saidcompensating element offsets thermal expansion of said resonant opticalcavity, wherein said first and second optical elements comprise firstand second reflectors, said compensating element coupled to at least oneof said reflectors, said compensating element configured such thatthermal expansion of said compensating element maintains a substantiallyconstant integer number of half wavelengths within said resonant opticalcavity.
 20. The laser apparatus of claim 19, wherein said firstreflector is tunable, said compensating element coupled to said firstreflector.
 21. The laser apparatus of claim 20, further comprising abase, said compensating element coupled to said base.
 22. The laserapparatus of claim 21, wherein said second reflector comprises a facetof a gain medium associated with said resonant optical cavity.
 23. Thelaser apparatus of claim 19, wherein said first optical element is atunable feedback element, said compensating element coupled to saidtunable feedback element.
 24. The laser apparatus of claim 23, furthercomprising a drive system coupled to said compensating element and saidtunable feedback element.
 25. The laser apparatus of claim 23, whereinsaid second optical element comprises a reflective facet of a gainmedium associated with said resonant optical cavity.
 26. The laserapparatus of claim 24, further comprising a base, said drive systemcoupled to said base.
 27. A laser apparatus comprising: (a) means fordefining a resonant optical cavity, said means for defining saidresonant optical cavity comprises first and second reflectors; and (b)thermal compensating means for offsetting thermal expansion of saidresonant optical cavity, said thermal compensating means coupled to atleast one of said reflectors and configured such that thermal expansionof said compensating element maintains a substantially constant integernumber of half wavelengths within said resonant optical cavity.
 28. Thelaser apparatus of claim 27, wherein said first optical elementcomprises tunable feedback means for selecting a wavelength, saidthermal compensating means coupled to said tunable feedback means. 29.The laser apparatus of claim 27, further comprising drive means fortuning said tunable feedback means, said thermal compensating meanscoupled to said drive means.
 30. A method for thermally stabilizing aresonant optical cavity, comprising: (a) positioning first and secondoptical elements to define said resonant optical cavity; and (b)offsetting thermal expansion of said resonant optical cavity bythermally compensating at least one of said optical elements, whereinsaid offsetting said thermal expansion comprises maintaining asubstantially constant integer number of half wavelengths within saidresonant optical cavity.
 31. The method of claim 30, wherein saidoffsetting said thermal expansion comprises coupling a compensatingelement to said at least one optical element, said compensating elementconfigured such that thermal expansion of said compensating elementoffsets thermal expansion of said resonant optical cavity.
 32. Themethod of claim 30, further comprising tuning at least one of saidoptical elements.
 33. The method of claim 32, wherein said offsettingsaid thermal expansion comprises coupling said compensating member to adrive system, said drive system configured to tune said at least oneoptical element.